Among the pollutants in combustion effluents are nitrogen oxides, referred to as a group as NO.sub.x. A number of strategies have been developed for reducing NO.sub.x levels, prominent among which is selective non-catalytic reduction (SNCR), disclosed for example by Lyon in U.S. Pat. No. 3,900,554 and by Arand et al in U.S. Pat. Nos. 4,208,386 and 4,325,924. Briefly, these patents disclose that ammonia (Lyon) and urea (Arand et al) can be injected into hot combustion gases to selectively react with NO.sub.x and reduce it to diatomic nitrogen and water.
The attainment of consistent, high reductions in NO.sub.x is a matter of considerable engineering and chemistry. These gas-phase SNCR reactions typically involve NO.sub.x levels of 100 to 1500 parts per million and either urea or ammonia at from one to three times the amount stoichiometrically required. Thus, the reaction requires mating of the reactive materials in high dilution, and typically starts with the NO.sub.x -reducing materials in aqueous droplets. The NO.sub.x -reducing material must be dispersed uniformly and continuously throughout the gas stream being treated to achieve contact with the NO.sub.x molecules in the temperature range effective for reaction, e.g., from 1600.degree. to 2000.degree. F.
Selective catalytic reduction (SCR) is similar to SNCR, but entails the use of a catalyst and operates at lower temperatures, generally within the range of from 250.degree. to 900.degree. F. See in this regard U.S. Pat. Nos. 3,032,387 and 3,599,427. The use of catalysts is effective but is sensitive to particulates and sulfur compounds and increases initial and operating costs in many situations.
Consistency in NO.sub.x reduction, especially while maintaining low levels of ammonia slip, is made even more difficult by the fact that the temperature across any plane varies significantly at any given time and shifts with changes in rate of combustion (i.e., load) which is common for boilers used in power generation and other combustors. To maximize NO.sub.x reduction, the art has developed to the state where chemicals can be injected in stages (U.S. Pat. No. 4,777,024 to Epperly et al), with variation in location of injection and chemical formulation as is necessary to meet the temperature and compositional variations in the gas stream being treated (U.S. Pat. No. 4,780,289 to Epperly et al). All piping, pumps, nozzles and associated equipment must be kept clean and clear for the objectives to be met. Frequent draining, flushing and washing are not possible without severe consequences.
In copending, commonly-assigned U.S. patent application Ser. No. 07/576,424, there is disclosed a low-cost composition for reducing nitrogen oxides which improves delivery of active chemicals to a high temperature zone by reducing the tendency of lines and nozzles to clog or otherwise become obstructed. As part of that disclosure, there are identified several sequestering agents and antiscalants to mitigate the effects of water hardness.
As part of work described in copending, commonly-assigned U.S. patent application Ser. No. 07/770,857, it was found that the nitrogen-based NO.sub.x -reducing agents which release the amidozine radical, produce a greater scaling problem than might ordinarily have been expected. This was found to be especially true where used with dilution water which has significant hardness such as calcium, magnesium and carbonate.
The reduction of scale in cooling towers by treating the cooling water has been the subject of study for years and has resulted in a large number of patents identifying a wide variety of chelants and threshold inhibitors.
Chelation is the binding between an inhibitor and a metal ion (e.g., Ca, Mg, Fe ions) at two or more sites. Among the known chelants are ethylene diamine tetracetic acid (EDTA), nitrilotriacetic acid (NTA), N-hydroxy ethyl ethylene diamine tetracetic acid, hydroxyethylene diamine triacetic acid (HEDTA), citric acid, diethylenetriamine pentacetic acid, gluconic acid, tartaric acid, glucoheptonic acid, and the water-soluble salts of these. The complexation of a metal ion with a chelant scale inhibitor results in dissolution of the metal ion. Chelation requires a 1:1 mole ratio of chelating agent to metal ion, and is therefore stoichiometric.
Threshold inhibition refers to the phenomenon where inhibitors prevent precipitation of mineral salts when added in amounts which are less than the amount of the scaling ion. Threshold inhibitors are typically viewed as acting through particle dispersion by steric stabilization and electrostatic repulsion, and retarding crystal growth by adsorption onto and blocking active growth sites. The ratio of threshold inhibitor to scaling ions is generally much smaller and is substoichiometric. Typically, threshold inhibitors (e.g., polymers, phosphonates) are applied at a dosage ratio of 1:10,000 to 1:2 ppm active inhibitor/ppm total hardness or particulate matters such as silt, clay or precipitate.
Representative of the patents disclosing compositions for treating cooling water are: Canadian Patent 1,117,395, to L. Dubin and J. A. Baumbach which teaches the use of phosphonocarboxylic acids and certain acrylic acid polymers; U.S. Pat. No. 3,663,448 to P. H. Ralston which teaches the use of combinations of amino phosphonate compounds and polyacrylic acid derivatives; U.S. Pat. No. 3,890,228 to C. M. Hwa, D. G. Cuisia and J. S. Gray, which teaches the use of combinations of polyacrylates and polymethacrylates with organo-phosphonic acids; U.S. Pat. Nos. 4,801,388 and 4,919,821, D. W. Fong, J. E. Hoots and J. Kneller which disclose various hydrocarbon polymers containing amido functionalities prepared by postpolymerization derivatization; and U.S. Pat. Nos. 4,752,443 and 4,923,634 to J. E. Hoots, D. A. Johnson, D. W. Fong and J. F. Kneller which teach a composition comprising a water-soluble inorganic phosphate capable of inhibiting corrosion in an aqueous alkaline environment and a hydrocarbon polymer containing an N-substituted acrylamide polymer with a specific structure.
Waste waters, such as blowdown water from cooling tower reservoirs treated with chemicals of the above and other types, often have solids concentrations which stress the effectiveness of scale control additives employed therein. Reducing the solids concentration in these waste waters requires either chemical or physical separation, both of which add costs. It would be desirable to enable replacement of at least a portion of the waste waters with fresh make-up water without either discharging the waste water or incurring costs required for treatment to reuse the water.